148 NEW ZEALAND NATURE-STUDY BOOK 
further back than the pectoral. The paired fins correspond 
to the limbs of other vertebrate animals, though differing 
considerably from these in structure. 
The unpaired fins are the dorsal or back fins, the anal 
fin placed on the under surface of the body and behind 
the pelvic fins, and the tail fin. These (dorsal, anal, and 
tail) are sometimes spoken of as the median fins, because 
they occur in the plane which divides the fish into two 
equal and similar portions. 
A fin is an expansion of the skin supported on a number 
of cartilaginous or bony rays, connected by muscles by 
means of which they can be expanded, folded up, or 
moved about. In many cases the fin-rays project beyond 
the fin-membrane and end in sharp stiff points, which furnish 
a formidable means of defence. The tail fin is an expansion 
of the skin covering the true tail, which consists of solid bone 
or of cartilage. In most fishes the fin-membrane is expanded 
into two equal and similar lobes—in the Shark and a few 
others the lobes are unequal. 
The eyes are well in front of the head and in most forms 
near the upper surface. These organs are large, circular, 
and covered with a horny transparent substance which pro- 
tects them from injury. There are no eyelids, the water 
which continually washes the eyes rendering eyelids 
unnecessary. 
The head is covered with a number of hard bony 
plates, one of which the operculum, or gill-cover, closes 
the opening leading to the gill-arches. There are of course 
two opercula, one on each side of the head. 
The gills and their uses. Fish, in common with 
all other animals, breathe air; but the air they require 
is dissolved in the water in which they live. Hf a 
glass vessel be filled with water and _ placed over 
the flame of a bunsen burner or a spirit lamp, bubbles 
